Photovoltaic back contact

ABSTRACT

A method to preparing Cadmium telluride surface before forming metal back contact is disclosed. The method can include removing carbon from Cadmium telluride surface.

CLAIM OF PRIORITY

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/241,606, filed on Sep. 11, 2009, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

This invention relates to a method of preparing a surface before formingmetal back contact for solar modules.

BACKGROUND

In order to create electrical contact to a surface of a photovoltaicdevice, the back contact layer can include metal.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of a photovoltaic device having multiple layersbefore preparing cadmium telluride surface.

FIG. 2 is a schematic showing the reactive ion etch process of preparingcadmium telluride surface.

FIG. 3 is a schematic of a photovoltaic device having multiple layerswith a metal back contact.

DETAILED DESCRIPTION

Photovoltaic modules, devices, or cells, can include multiple layers (orcoatings) created on a substrate (or superstrate). For example, aphotovoltaic device can include a barrier layer, a transparentconductive oxide (TCO) layer, a buffer layer, and a semiconductor layerformed in a stack on a substrate. Each layer may in turn include morethan one layer or film. For example, the semiconductor layer can includea first film including a semiconductor window layer, such as a cadmiumsulfide layer, formed on the buffer layer and a second film including asemiconductor absorber layer, such as a cadmium telluride layer formedon the semiconductor window layer. Additionally, each layer can coverall or a portion of the device and/or all or a portion of the layer orsubstrate underlying the layer. For example, a “layer” can include anyamount of any material that contacts all or a portion of a surface. Inorder to electrically connect a photovoltaic device, the back contactlayer can include metal. For cadmium telluride (CdTe) solar cells, thepresence of carbon residue and oxide on the cadmium telluride surfacebefore forming metal back contact can affect the photovoltaic deviceperformance.

A photovoltaic device can include a transparent conductive oxide layeradjacent to a substrate and layers of semiconductor material. The layersof semiconductor material can include a bi-layer, which may include ann-type semiconductor window layer, and a p-type semiconductor absorberlayer. The n-type window layer and the p-type absorber layer may bepositioned in contact with one another to create an electric field.Photons can free electron-hole pairs upon making contact with the n-typewindow layer, sending electrons to the n side and holes to the p side.Electrons can flow back to the p side via an external current path. Theresulting electron flow provides current which, combined with theresulting voltage from the electric field, creates power. The result isthe conversion of photon energy into electric power.

Cadmium telluride has been a desired material for solar cell absorberlayer because of its optimal band structure and low cost manufacturing.In order to electrically connect a photovoltaic device, the back contactlayer can include metal. For cadmium telluride solar cells, the backcontact composition is critical to device performance. The surface ofthe cadmium telluride absorber layer can be prepared prior to forming ametal back contact adjacent to the cadmium telluride layer. However,during device manufacture, solar cells can absorb carbon-based materialfrom the plant environment. The device surfaces can also becomeoxidized. This will change the composition of the back contact. As aresult, the presence of carbon residue and oxide on the cadmiumtelluride surface before forming metal back contact can affect thephotovoltaic device performance. A method of cleaning cadmium telluridesurface before forming metal back contact can be developed to addressthis problem.

The method of cleaning a cadmium telluride surface can includecontacting a cleaning agent with the cadmium telluride surface. In oneaspect, carbon or oxygen can be removed from a semiconductor surfacebefore forming a metal back contact adjacent to the photovoltaic devicelayer. The carbon can be a carbon residue. The carbon can be a carbonlayer adjacent to the photovoltaic device layer. The photovoltaic devicelayer can be the window layer. The photovoltaic device layer can includecadmium telluride. The carbon or carbon-containing material can beremoved by contacting a cleaning agent to a portion of the carbon orcarbon-containing material. The cleaning agent can be contacted to thecarbon or carbon-containing material in a location adjacent to thephotovoltaic device layer.

In another aspect, a process for manufacturing a photovoltaic device caninclude forming a transparent conductive oxide layer adjacent to asubstrate, forming a semiconductor window layer adjacent to thetransparent conductive oxide layer, forming a semiconductor absorberlayer adjacent to the semiconductor window layer, wherein thesemiconductor absorber layer comprises cadmium telluride and acarbon-containing material. The process can further include contacting acleaning agent to the carbon-containing material. The step of contactinga cleaning agent to the carbon-containing material can remove a portionof the carbon-containing material from the absorber layer. The absorberlayer can have a carbon-containing layer and the step of contacting acleaning agent with the absorber layer surface can alter the thicknessof the carbon-containing material. After the carbon or carbon-containingmaterial is removed, the thickness of an oxide layer can be adjustedbefore forming a back contact layer adjacent to the absorber layersurface.

The cleaning agent can include an acidic solution. The acidic solutioncan have a pH value in the range of about 3 to about 5. The acidicsolution can include an organic acid. The acidic solution can include anacid such as aspartic acid, citric acid, gluconic acid, glutamic acid,maleic acid, oxalic acid, propionic acid, salicylic acid, and tartaricacid, or combinations of these acids, or any other suitable acid.

The cleaning agent can include an alkaline solution. The alkalinesolution can have a pH value higher than about 9. The pH value of thealkaline solution can be adjusted by an amine compound. Examples of suchamine compounds include ethylene diamine, tetra-alkyl ammonium salt,isopropanolamine, or isopropylhydroxylamine.

The cleaning agent can include a surfactant. For example, the surfactantcan include a cationic surfactant, an anionic surfactant or a nonionicsurfactant.

The cleaning agent can include a chelating agent. The chelating agentcan be any suitable ion-binding material. Examples of chelating agentsthat can be used include ethylene diamine, gluconic acid,isopropanolamine, isopropylhydroxylamine, dicarboxymethylglutamic acid,ethylenediamine-N,N′-disuccinic acid, or ethylenediaminetetraaceticacid.

The cleaning agent can include any suitable oxidizing material. Forexample, the cleaning agent can include ferric ammonium citrate, ferricchloride, ferric nitrate, ammonium cerium nitrate, N-bromosuccinamide,copper chlorate, pyridinium tribromide, or trifluoro-peracetic acid.

Contacting the cleaning agent to the carbon-containing material caninclude a dry process, for example, a process in the absence of water orother solvent. The dry process can include a reactive ion etch process.The dry process can include a plasma enhanced etch process with areactive gas. The dry process can include a reactive ion etch process.The dry process can include a plasma enhanced etch process with areactive gas. The reactive gas can include a reducing gas, oxidizing gasor any suitable gas. The reactive gas can include hydrogen.

The process of removing the carbon or carbon-containing material fromthe photovoltaic device layer can further include the step of removingoxide from the device layer surface, prior to forming the back contact.

The process of removing the carbon or carbon-containing material fromthe photovoltaic device layer can further include a material depositionstep with a reactive gas. The deposited material can include carbon,carbon-containing species, or any other suitable material. Thedeposition process can be a plasma enhanced chemical vapor deposition(PECVD) or any other suitable deposition technique or chemical vapordeposition technique. The back contact can be oxidized, reduced, ormaterial deposited with a reactive gas.

The process of manufacturing a photovoltaic device can further include astep of adjusting the thickness of an oxide layer after contacting acleaning agent to the carbon-containing material.

The process of manufacturing a photovoltaic device can further include astep of adjusting the cadmium telluride ratio on the back contactsurface. The back contact can include a metal and a metal oxide. Theprocess of manufacturing a photovoltaic device can further include astep of adjusting the metal to metal oxide ratio in the back contact.

In another aspect, a multilayer structure can include a substrate, atransparent conductive oxide layer adjacent to the substrate, asemiconductor window layer including cadmium sulfide adjacent to thetransparent conductive oxide layer, a semiconductor absorber layerincluding cadmium telluride and a carbon-containing material adjacent tothe semiconductor window layer, and a cleaning agent adjacent to thesemiconductor absorber layer.

Referring to FIG. 1, photovoltaic device 100 can include transparentconductive oxide layer 120 deposited adjacent to a substrate 110.Transparent conductive oxide layer 120 can be deposited on substrate 110by sputtering or evaporation or any other appropriate method. Substrate110 can include any suitable substrate material, including glass, suchas soda-lime glass. Transparent conductive oxide layer 120 can includeany suitable transparent conductive oxide material, including a cadmiumstannate, an indium-doped cadmium oxide, or a tin-doped indium oxide.Semiconductor window layer 130 can be deposited adjacent to transparentconductive oxide layer 120. Semiconductor window layer 130 can bedeposited adjacent to transparent conductive oxide layer 120, which canbe annealed. Semiconductor window layer 130 can include any suitablewindow material, such as cadmium sulfide, and can be formed by anysuitable deposition method, such as sputtering or vapor transportdeposition. Semiconductor absorber layer 140 can be deposited adjacentto semiconductor window layer 130. Cadmium telluride absorber layer 140can be deposited on semiconductor window layer 130. Cadmium tellurideabsorber layer 140 can have surface 141 facing up. During manufacture,solar cells can absorb carbon-based material from the plant environmentand slowly air oxidizes. All these factors can result in presence ofcarbon residue and oxide 150 on cadmium telluride surface 141 beforeforming metal back contact.

Cadmium telluride surface 141 can be treated so oxides and carbonresidues are removed. This can be accomplished with a chemical treatmentthat is oxidizing in an acidic or alkaline solution. In someembodiments, the acidic solutions can have pH value in the range ofabout 3.0 to about 5.0. The acidity can be adjusted using weak organicacids such as aspartic acid, benzoic acid, citric acid, gluconic acid,glutamic acidmaleic acid, oxalic acid, propionic acid, salicylic acid,tartaric acid, or any suitable mixtures of these acids. In someembodiments, the alkaline solutions can have pH value higher than about9.0. The pH value can be adjusted with any number of amine compoundssuch as ethylene diamine, tetra-alkyl ammonium salts, isopropanolamine,isopropylhydroxylamine, or any suitable mixtures of these compounds.

The chemical treatment can include a cleaning agent which can becontacted to the carbon reside 150, adjacent to a photovoltaic devicelayer. The cleaning agent can be contacted to carbon residue 150adjacent to semiconductor absorber layer 140. The cleaning agent maycontain surfactants to aid in the removal of larger particles on cadmiumtelluride surface 141. The surfactants can include cationic, anionic, ornonionic surfactants.

The cleaning agent or solution which can be contacted to the carbonadjacent to semiconductor absorber layer 140 may contain chelatingagents which selectively bind Cd or Te. Examples of chelating agentsinclude compounds such as ethylene diamine, gluconic acid,isopropanolamine, isopropylhydroxylamine, dicarboxymethylglutamic acid,ethylenediamine-N,N′-disuccinic acid (EDDS), ethylenediaminetetraaceticacid (EDTA), or any other suitable chelating agent. Cleaning agents caninclude ferric ammonium citrate, ferric chloride, ferric nitrate,ammonium cerium nitrate, N-bromosuccinamide, copper chlorate, pyridiniumtribromide, trifluoro-peracetic acid, or any other suitable oxidizingagent.

In some embodiments, the oxides and carbon residues can be removedthrough a dry process. The dry process can include reactive ion etching(RIE), vapor phase etching, or other suitable dry etch process.

Referring to FIG. 2, in some embodiments, for a reactive ion etchprocess of preparing cadmium telluride surface in RIE system 200,photovoltaic device 100 can be placed inside reactor chamber 210 withcadmium telluride surface 141 (FIG. 1) facing up, wherein one or moregases 260 are introduced into reactor chamber 210 via diffuser nozzle270. Upper electrode 220 and lower electrode 230 can be positionedoppositely in chamber 210. Lower electrode 230 can also used to holdphotovoltaic device 100. A plasma is struck in the gas mixture using RFpower source 240, breaking the gas molecules into ions. RF insulator 250can be included in the chamber wall to isolate RF input. The ions can beaccelerated towards, and reacts at, the surface of the material beingetched, forming another gaseous material 280 that can be pumped out.This is known as the chemical part of reactive ion etching. There isalso a physical part which is similar in nature to the sputteringdeposition process. If the ions have high enough energy, they can knockcarbon and oxide out of cadmium telluride surface 141 (FIG. 1) to beetched without a chemical reaction. It can be a complex task to developa dry etch process that balances chemical and physical etching, sincethere are many parameters to adjust. By changing the balance it ispossible to influence the resulting composition and roughness of thecleaned cadmium telluride surface 141 (FIG. 1).

Vapor phase etching can also be a dry etching solution, which can bedone with simpler equipment than what RIE requires. In this processphotovoltaic device 100 can be placed inside a chamber to be etched, inwhich one or more gases are introduced. The material to be etched isdissolved at the surface in a chemical reaction with the gas molecules.In some embodiments, the oxides and carbon residues can be removedthrough a plasma enhanced dry process with reactive gases such ashydrogen.

As discussed, the treatment can be a wet treatment, dry etch process, orany suitable combination of both. In some embodiments, the treatment canbe used to erase any compositional changes caused by previous processingsteps. In this manner, all carbon residue can be removed. Furthermore,the Cd/Te ratio and the metal oxide layer thickness can be adjusted tocontrol the back contact composition.

Referring to FIG. 3, after the steps of preparing cadmium telluridesurface and forming metal back contact on cadmium telluride surface 141,a photovoltaic device 300 can include a transparent conductive oxidelayer 120 deposited adjacent to a substrate 110. Transparent conductiveoxide layer 120 can be deposited on substrate 110 by sputtering orevaporation or any other appropriate method. Substrate 110 can includeany suitable substrate material, including glass, such as soda-limeglass. Transparent conductive oxide layer 120 can include any suitabletransparent conductive oxide material, including a cadmium stannate, anindium-doped cadmium oxide, or a tin-doped indium oxide. Semiconductorwindow layer 130 can be deposited adjacent to transparent conductiveoxide layer 120. Semiconductor window layer 130 can be depositedadjacent to transparent conductive oxide layer 120, which can beannealed. Semiconductor window layer 130 can include any suitable windowmaterial, such as cadmium sulfide, and can be formed by any suitabledeposition method, such as sputtering or vapor transport deposition.Cadmium telluride absorber layer 140 can be deposited adjacent tosemiconductor window layer 130. Cadmium telluride absorber layer 140 canbe formed by any suitable method, such as sputtering or vapor transportdeposition. Back contact 160 can be formed adjacent to cadmium tellurideabsorber layer 140. Back contact 160 can be formed on treated surface141 of cadmium telluride absorber layer 140. In order to electricallyconnect a photovoltaic device, back contact 160 can include any suitablemetal, such as molybdenum or copper.

In certain embodiments, a photovoltaic device can have a back contactlayer which includes a back contact material to have better performance.The back contact material can include any suitable material. Forexample, the back contact material can include any other suitablematerial such as tellurium, selenium, calcium, lead, mercury orgraphite.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Itshould also be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention.

What is claimed is:
 1. A method of cleaning a cadmium telluride surface,comprising contacting a cleaning agent with the cadmium telluridesurface.
 2. The method of claim 1, wherein the cleaning agent comprisesan acidic solution.
 3. The method of claim 2, wherein the acidicsolution has a pH value in the range of about 3 to about
 5. 4. Themethod of claim 2, wherein the acidic solution comprises an organicacid.
 5. The method of claim 2, wherein the acidic solution comprises anacid selected from the group consisting of aspartic acid, citric acid,gluconic acid, glutamic acid, maleic acid, oxalic acid, propionic acid,salicylic acid, and tartaric acid.
 6. The method of claim 1, wherein thecleaning agent comprises an alkaline solution.
 7. The method of claim 6,wherein the alkaline solution has a pH value higher than about
 9. 8. Themethod of claim 6, wherein the pH value of the alkaline solution isadjusted by an amine compound.
 9. The method of claim 8, wherein theamine compound is selected from the group consisting of ethylenediamine, tetra-alkyl ammonium salt, isopropanolamine, andisopropylhydroxylamine.
 10. The method of claim 1, wherein the cleaningagent comprises a surfactant.
 11. The method of claim 10, wherein thesurfactant comprises a cationic surfactant.
 12. The method of claim 10,wherein the surfactant comprises an anionic surfactant.
 13. The methodof claim 10, wherein the surfactant comprises a nonionic surfactant. 14.The method of claim 1, wherein the cleaning agent comprises a chelatingagent.
 15. The method of claim 14, wherein the chelating agent isselected from the group consisting of ethylene diamine, gluconic acid,isopropanolamine, isopropylhydroxylamine, dicarboxymethylglutamic acid,ethylenediamine-N,N′-disuccinic acid, and ethylenediaminetetraaceticacid.
 16. The method of claim 1, wherein the cleaning agent comprisesferric ammonium citrate, ferric chloride, ferric nitrate, ammoniumcerium nitrate, N-bromosuccinamide, copper chlorate, pyridiniumtribromide, or trifluoro-peracetic acid.
 17. A method of manufacturing aphotovoltaic device comprising the steps of: forming a transparentconductive oxide layer adjacent to a substrate; forming a semiconductorwindow layer adjacent to the transparent conductive oxide layer; forminga semiconductor absorber layer adjacent to the semiconductor windowlayer, wherein the semiconductor absorber layer comprises cadmiumtelluride and a carbon-containing material; contacting a cleaning agentto the carbon-containing material, thereby removing a portion of thecarbon-containing material from the cadmium telluride layer; and forminga back contact layer adjacent to the absorber layer surface.
 18. Themethod of claim 17, wherein the step of contacting a cleaning agent tothe carbon-containing material alters the thickness of thecarbon-containing material.
 19. The method of claim 17, furthercomprising adjusting the thickness of an oxide layer after contacting acleaning agent to the carbon-containing material.
 20. The method ofclaim 17, further comprising adjusting the cadmium telluride ratio onthe back contact surface.
 21. The method of claim 17, wherein the backcontact comprises a metal and a metal oxide.
 22. The method of claim 21,further comprising adjusting the metal to metal oxide ratio in the backcontact.
 23. A multilayer structure comprising a substrate; atransparent conductive oxide layer adjacent to the substrate; asemiconductor window layer comprising cadmium sulfide adjacent to thetransparent conductive oxide layer; a semiconductor absorber layercomprising cadmium telluride and a carbon-containing material adjacentto the semiconductor window layer; and a cleaning agent adjacent to thesemiconductor absorber layer.